Raster imaging output devices depict images as a series of on and off pixels within a two-dimensional pixel array (of vertical columns or lines and horizontal rows), referred to herein as a dot matrix, addressed in a dot-serial, raster scanning pattern. Image resolution, and thus clarity, conformity and edge definition, is determined in part by the size of the pixels that make up the image. Some raster imaging output devices have resolutions up to 300-dots per inch (dpi) along a vertical axis and 600-dpi along a horizontal axis, the latter of which conventionally is the raster scan axis. It will be understood that such pixel images may consist of text, graphics or any abstract forms characterized by an instantaneous pixel image defined by whether each pixel is on or off. The higher the resolution of the pixel image, the higher the quality of the output.
One such raster imaging output device is the inkjet printer. An inkjet printer typically has substantially equal vertical and horizontal resolutions, e.g. 300-dpi, due to the round character of individual suspended ink droplets that form the pixel image on paper. Although it is possible to increase the effective resolution of an inkjet printer along the horizontal, raster scan, axis by simply energizing, or firing, the ink jets at a higher rate, such would require substantial head redesign effort and would result in substantially higher cost. It is possible to increase effective resolution by slowing down the rate at which the print head moves relative to the paper, but of course this would result in lower printer throughput. Either attempt to increase raster scan axis resolution would result in the placement of excess ink on the paper due to too much ink droplet overlap.
Most printers inherently have a maximum firing repetition rate, or a minimum cycle time, which if exceeded unduly stresses the print head's electronic and/or mechanical assemblies. Such a maximum refiring rate is one of the factors that determines the resolution of the printer along the raster scan axis. Some printers impose these physical constraints on received pixel image data that would otherwise cause the print head to refire too rapidly. They do so by simply ignoring certain pixel data, e.g. every other on pixel in a given raster scan row of a pixel image might be ignored. Effectively, the horizontal pixel density of the printed output is reduced.
Such techniques reduce the quality of the output of raster output devices. This is because turning off pixels at an image boundary results in undesirably stairstepped or jagged edges, where a smooth edge is desired. Such unintended jagged edges in raster imaging devices will be referred to herein as jaggies. Lower quality also results when logical pixels that happen to be aligned (vertically) in adjacent rows of the pixel image are ignored by the printer, which causes visible, vertical, "white" (or unprinted) gaps, in what should have been a solid "black" (or colored), printed image.